Paola Girdinio

Equivalent sources methods for the numerical evaluation of magnetic force with extension to nonlinear materials

In this paper the equivalent sources methods, written in terms both of the actual and of the external field, are used for the computation of the total force in two-dimensional axisymmetric problems. A Finite Element modeling of the magnetizing currents as field sources is presented in order to improve the formulations using the external field. The numerical accuracy of all the proposed formulae is thoroughly checked on linear and nonlinear test examples.

Time-Domain Implementation of Cooray–Rubinstein Formula via Convolution Integral and Rational Approximation

One of the most popular techniques to take into account the finite ground conductivity in the evaluation of the radial component of the electric field generated by a lightning return stroke is the Cooray-Rubinstein (CR) formula. As this formula is derived in the frequency domain, its application in time-domain simulation codes for the analysis of lightning-induced overvoltages on overhead lines might be challenging. In this paper, two methods allowing a simple time-domain implementation of the formula are discussed. The first one is an enhanced version of an algorithm recently proposed by two of the authors, while the second consists of a new technique, based on a suitable rational approximation of the impedance term appearing in the CR formula. This second approach is characterized by an intrinsic easiness in time-domain implementation and a considerable computational efficiency compared with conventional methods.

A field‐based inverse algorithm for the identification of different height lightning return strokes

An identification procedure for lightning return strokes is proposed. It makes it possible to reconstruct the return stroke height‐dependent attenuation function making use only of the measured vertical component of the electric field radiated by the lightning channel and it can be performed without any information about the channel base current, the measurement of which is generally very difficult and noisy. It is also shown that the method, with suitable modifications, can also be employed for the mathematical evaluation of the lightning channel height, which is a physical quantity a priori unknown. The approach has been validated by means of the numerical simulation of classical TL, MTLL and MTLE engineering models for a wide class of return strokes, with height ranging in the interval 15‐30km.

An identification procedure for lightning return strokes

Abstract In this paper an inverse procedure for the identification and the reconstruction of the waveform of the lightning return stroke current is presented. It is based on the acquisition of the vertical component of the electric field radiated by the discharge channel at different locations on the ground and it does not require any information about the channel base current. The approach has been validated by means of the numerical simulation of the classical Transmission Line, Modified Transmission Line Linear and Modified Transmission Line Exponential engineering models, showing good accuracy in all cases.

Some data on the AC breakdown strength of liquid helium in the millimetre gap range

Abstract In this paper the first results of an investigation aimed to obtain additional data on breakdown strength of liquid helium are presented; the work is performed under as controlled as possible conditions not too far removed from those relevant to practical engineering applications, by means of a purpose-built helium cryostat whose main features are briefly described. To get results suitable for direct engineering use, a first group of tests was performed with IEC normalized spherical cap electrodes, investigating the statistical behaviour of breakdown strength as a function of gap setting and test conditions. A second group of tests was performed with cylindrical electrodes to get data on the amount of the area effect on the statistical features of breakdown, also as function of test conditions. Finally, a statistical analysis of the obtained data is performed and results are discussed.

Equivalent source methods for 3-D force calculation with nodal and mixed FEM in magnetostatic problems

In this paper, the application of equivalent source methods for the numerical computation of total and local forces in three-dimensional magnetostatics is proposed. A nodal two-scalar potential formulation and a mixed face-edge formulation are used to solve the field problem and to derive force values, which are then compared to analytical results. The numerical accuracy of the proposed methods, in connection both to the used formulation and to the level of refinement of the mesh, is analyzed and discussed. It is shown that results coming from the mixed solution are more accurate than those obtained with the nodal one.

Electromagnetic plane wave scattering from building surfaces

Purpose – To provide a detailed investigation about methods commonly used for the computation of high‐frequency electromagnetic fields in the vicinity of obstacles that can reflect or diffract them. This is useful to create an elementary block that can be used to evaluate with a high accuracy the propagation of high‐frequency electromagnetic waves in real urban environment.Design/methodology/approach – The approach is based on a realistic application of the asymptotic theory of the uniform theory of diffraction. Therefore, the effect of material roughness and its electromagnetic properties on the reflection are taken into account.Findings – Provides information about the mechanisms involved in electromagnetic field propagation in urban environment, and the relative importance of each one.Research limitations/implications – In urban environment the buildings obviously have finite dimensions. The diffraction equations examined in the paper are strictly valid only in the case of infinite wedges; therefore, t...

A statistical analysis of the AC dielectric strength of liquid nitrogen with IEC spherical electrodes under controlled conditions

Abstract In this paper an analysis of the breakdown strength of liquid nitrogen with IEC spherical electrodes and adjustable gap setting under as controlled as possible conditions is presented. The analysis is performed on commercial quality liquid nitrogen in the millimetre gap range, to collect data on the reduction of performances with respect to the technically pure liquids in a range of gaps not too far from those of practical applications, and with normalized electrodes to simplify technical comparison with other dielectric liquids. With a specially developed apparatus several series of tests are performed, to get additional informations on the gap effect in the millimetre gap range, and a thorough investigation on the influence of stirring between tests is carried out. Finally, a statistical analysis of the obtained results is performed to get further hints also on the conditioning effect as well as on the influence of sample size, and all the obtained results are compared and discussed.

Hybrid deterministic/stochastic fuzzy methods for the optimization of electromagnetic devices

Discusses the combined use of a modified version of the global optimization technique simulated annealing and a deterministic optimizer based on Shor’s method. Describes the features of the proposed technique and reports on some results regarding a standard benchmark problem.

First results of computer aided identification of model parameters for field-enhanced impurity motion in dielectric liquids

Abstract To allow a more reliable use of a previously developed computer model of the motion of impurity particles in a dielectric liquid under the action of a time-varying electric field, a sophisticated data acquisition system has been devised and built at the Electrical Engineering Department of the University of Genoa. The apparatus consists of an advanced optical system which allows a real time acquisition of the film frames. The images are then analysed, digitized and stored into an on-line computer by means of a fast memory processor. The data are then scanned by speecially developed software to collect the kinematic quantities necessary to evaluate model parameters. In the paper, the above apparatus is described, and first results of the automatic analysis are shown and compared with experimental results. Furthermore, some consequent improvements of the model are reported and discussed.